1960ish Liston maybe, 1964ish Ali probably. Eventually Ali begins to age where Louis doesn't, so he gets revenge on him eventually. I mean he's going to be a fierce champion /contender until the end of the 21st century. He's going to get "modern nutrition", he's going to have more experience against more ATGs and champions than anyone in history, he's going to be training with (probably) multiple amazing trainers. Edit: Just saw that he can't get any better, he's still going to be gaining experience as the decades wear on, so I'm keeping that advantage. He's going to be a constant threat through each era.
Potentially he could beat everybody, I think he and Blackburn would work Liston and Ali out pretty methodically.
HW Champion and/or Contender up until the 90s or so, after that he is then fighting at L-HW just like every other fighter his Size & Statute then and since.
I wonder if he would be jumping up and down from Cruiserweight as it suits him or if he has a bad matchup in an era that he can't beat. Like if he lost to Lennox Lewis, he just goes down to cruiser and crushes the titilists down there, bidding his time to come back up (he would be smashing guys like Nate Miller, Adolpho Washington, Uriah Grant etc)
From the start of his championship run until the 1960's if he didn't age he beats everyone. 1961- 63 he loses to prime Liston. 1964 he'd lose to Ali. About 68-69 Ali of course is forced out, he beats everyone with the exception of Joe Frazier. 1972- 73 he meets Foreman, loses. Ali after 1975 he beats more than likely. 1977 he destroys Leon Spinks, meets Larry Holmes in 78. Tough fight, but he beats Holmes. If he stays away from Jimmy Young , everyone else he matches up well against. He'd need to be very careful and focused against Lyle, Shavers, and Cooney. The fat heavy years of the early 80' s he should be fine. Page, Tubbs, Dokes, Bey, Witherspoon, wouldn't be a serious threat. Smith, And Tucker were hugh men, but very predictable. Tyson would be a very dangerous opponent for Louis. I don't believe he'd survive him. As long as he doesn't fight Douglas a week or so past his mother's death , he's no real threat. Holyfield, tough fight for Louis, but a tougher fight for Holyfield. Though he believes he'd beat Louis, he more than likely fights with his heart and not his brain. Louis would eventually catch him with something, he'd try to retaliate, gets K'oed. Bowe in shape beats Louis, but would take a lot of punishment. Lewis, would be smart enough not to get into a shoot out with Louis, he wins, but he'd need to be completely focused. The Kleichkos are dangerous because of their hugh size advantages, but they were very cautious types, that would take away those advantages. Straight up European style fighters are very predictable. That's very risky against a fighter that could counter with the speed, and power of Louis. Fury could win, if he stays sharp and focused, his sheer height, weight, and reach advantages, plus being surprisingly light on his feet would serve him well against Louis . Though the mistakes that got him dropped by Wilder, would get him K'oed by Louis . Wilder, lol. Please. He destroys.
The Tyson fight is interesting, I can envision Tyson ambushing him early just as easily as I can imagine Louis taking Tyson into the later rounds and stopping him. Louis will have 50 years of experience by this point lol Post prison Tyson takes a beating
more likely S-MW to L-HW, that is Louis' Size - Height & Stature, just like all the 'other' modern Super Middles & Light Heavies.
His peak was ~207 lbs/1942, although he was 198.5 rematching Schmelling. It is dubious that he would be effective at LHW, that would be pretty drained, CW definitely.